A coating film formed from urethane coating composition using a polyisocyanate as a curing agent is excellent in chemical resistance, flexibility and the like. Particularly, when a polyisocyanate obtained from an aliphatic or alicyclic diisocyanate is used, the coating film is further excellent in weatherability. Therefore, the polyisocyanate is used in the form of a room-temperature curable two-component urethane coating composition or a thermosetting one-component urethane coating composition in a wide variety of fields such as construction, heavy-duty coating, automobiles, industrial uses and there repairing.
Since the thermosetting one-component urethane coating composition does not react at room temperature and is excellent in storage stability, it is excellent in workability because the mixing of a polyol and a polyisocyanate immediately before use, which is required in the case of a two-component urethane coating composition, is not required. However, the curing temperature of the one-component urethane coating composition is high and has been desired to be decreased. The high baking temperature has limited coating on a material having low heat resistance, as well as been causing a lot of energy to be consumed.
There are several proposals for improving the low-temperature curability of a blocked polyisocyanate.
A certain degree of low-temperature curability was achieved by using a pyrazole compound as a blocking agent (Patent Document 1). However, unsmoothness, which was clearly visible to the naked eye, was formed on the surface of the coating film formed by using the compound as a curing agent, thereby sometimes providing a very poor appearance of the coating film.
In addition, a technique is proposed in which the number of the isocyanate functional groups is increased to improve the low-temperature curability of a blocked polyisocyanate (Patent Document 2). Although the technique achieved a certain degree of the low-temperature curability, the hardness of the cured coating film was insufficient in some cases.
Because the necessity of a high temperature in curing brings about increase in the generation of carbon dioxide gas and the energy cost, further low-temperature curability is desired.    PATENT DOCUMENT 1: European Patent No. 0159117    PATENT DOCUMENT 2: JP 08-325353 A